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Astron. Astrophys. 343, 983-989 (1999)

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2. Observations

The observations were obtained in June 1997 with the Vacuum Tower Telescope (VTT) at the Observatorio del Teide. The data analyzed here stem from disc centre of the Sun. Two single spectral scans across Na D2 with our FPI were selected. They are part of time sequences taken at different days. The data from two different days are chosen on purpose, to convince ourselves that the results are not singular in the sense that they are obtained from a special area on the Sun under special seeing conditions. The full analysis of the time sequences is laborious and is deferred to a subsequent contribution. The D2 line was chosen despite its blended core since it has stronger wings than D1, thus a stronger wavelength dependence of the height of formation of the wing intensities.

The FPI is described in Bendlin et al. (1992), Bendlin (1993), and Bendlin & Volkmer (1995). Referring especially to Fig. 3 in Bendlin et al. (1992) we comment here on its basic features and mention the changes from its normal setup: The telescope's primary focal plane (at F) is imaged into a plane at F´ in an optical laboratory of the VTT. In front of F´, a beam splitter takes out approximately 5% of the light which go into the broad-band channel. The remainder of the light goes straight into the narrow-band channel.

Broad-band channel. Here, an interference filter of 10 Å centred at Na D2 and a variable grey filter are followed by a CCD - we call it CCD1 - located in a plane congruent to F´.

Narrow-band channel. The focal plane F´ is re-imaged onto F" where another detector, CCD2, is mounted. The re-imaging is done in such a way that the FPI with its limited aperture can be placed at the image of the telescope's pupil. This position of the FPI gives a wavelength variation of the transmission (cf. Airy's formula) from the centre of the field of view to its corners. We neglect this in the data reduction for the present purposes since it amounts only to -16 mÅ in the raw images, and even less, -9 mÅ, in the remaining field of view after the restorations. The spacing of the FPI etalons was chosen to give 200 mÅ bandwidth (FWHM). Its finesse was approximately 35. To sort the orders of the FPI an interference filter with 3 Å FWHM, again centred at Na D2, was placed close to F´. This is different from the setup described in Bendlin et al. (1992) where a universal birefringent filter (UBF) with low transmissivity ([FORMULA] 10%) was used. (Actually, for regular use with high transmissivity ([FORMULA] 50%), a second FPI is now mounted instead of the UBF close to the first one.)

An image scale of 0:001 per pixel on the 384[FORMULA]286 Thomson chips of the slow-scan CCDs from LaVision was chosen. The simultaneous exposures of CCD1 and CCD2, the data transfer, and the variation of the spacing of the FPI etalons (to change the wavelength position of the passband) is done by computer control.

The scans across Na D2 consisted of 140 short exposure (6 ms) frames taken with both CCD1 and CCD2. After each 5 exposures the FPI transmission was shifted by 100 mÅ so that we have 28 wavelength positions around Na D2 in the narrow-band channel. Fig. 1 shows which intensities, relative to the quiet Sun continuum intensity, must be expected from the setup. The measured average frame intensities come close to the solid profile in Fig. 1.

[FIGURE] Fig. 1. Disc centre Na D2 profiles. Dash-dotted: obtained with a Fourier Transform Spectrometer by Brault & Neckel (1987); dot-dot-dot-dashed: after convolution with the (normalized) FPI transmission curve (dashed, Airy's formula); solid: after multiplication of the convolved profile with a Gaussian (dotted) of 3 Å FWHM to mimic the order sorting interference filter.

In addition, several data were taken for the reduction later on: darks, flat fields (with defocussed telescope), scans with a fixed pattern in the VTT's primary focus to correct for relative displacements and slightly different scales of the images taken with CCD1 and CCD2.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999